Differential Diagnosis of Chronic Posterolateral Elbow Dislocation

1. Congenital Posterolateral Radial Head Dislocation ⭐ (Most Important DD)

• The radial head dislocates posterolaterally — exactly the same direction as the most common traumatic dislocation.
• Presents in childhood or young adulthood with painless restricted forearm rotation and a lateral prominence.
• On X-ray: the radial head is hypoplastic/dome-shaped (not the normal cup shape), the radial neck is elongated, and there is often associated proximal radioulnar synostosis — none of these features occur in traumatic dislocation.
• Key distinction: congenital dislocation → underdeveloped/dome-shaped radial head + no history of injury; traumatic chronic dislocation → normal radial head morphology + definite mechanism of injury.
• Bailey and Love's Surgery, 28th ed.: "The dislocation is usually posterolateral, compared with the classic traumatic anterior dislocation. Some restriction of elbow joint movement and forearm rotation is noted."

2. Posterolateral Rotatory Instability (PLRI) — Recurrent Subluxation

• PLRI is caused by incompetence of the Lateral Ulnar Collateral Ligament (LUCL) and is the most common form of chronic elbow instability (80% of all elbow instability patterns).
• Patients present with a history of prior elbow dislocation treated nonoperatively, with recurrent giving way, pain, and a subjective sense of instability — rather than a fixed dislocation.
• The distinction is reducible vs. fixed: in PLRI the joint reduces spontaneously (especially in pronation), whereas in chronic dislocation the joint is permanently displaced and irreducible.
• Lateral pivot-shift test: supination + axial compression + valgus → posterior radial head subluxation with a palpable clunk on reduction.
• MRI shows LUCL pathology in ~50% of cases.
• Miller's Review of Orthopaedics 9th ed.: "Caused by incompetence of the LUCL; patients relate history of previous elbow dislocation treated nonoperatively."

3. Terrible Triad Injury (Missed/Chronic)

• Terrible triad = elbow dislocation + radial head fracture + coronoid process fracture (both posterolateral structures).
• If the dislocation component is missed acutely, the patient may present late with a "chronic dislocation" appearance — but associated fractures make this anatomically distinct and surgically more demanding.
• The LCL avulsion is typically from the lateral epicondyle (not the mid-substance).
• CT is mandatory to identify fracture fragments that alter surgical planning.
• Rockwood & Green 10th ed.: "An important consideration when evaluating the elbow dislocation is the 'terrible triad' when a posterior dislocation occurs with associated radial head and coronoid process fractures."

4. Monteggia Fracture-Dislocation (Chronic/Missed)

• Bado Type I (anterior radial head dislocation) is the classic, but Type II (posterior) and Type IV (posterolateral) can be missed and present late with a radial head that appears chronically displaced posterolaterally.
• Key distinction: there is always a proximal ulnar fracture (or malunion in chronic cases). The radial head dislocation cannot be maintained without the ulnar deformity — "the ulna always goes with the radius."
• In chronic Monteggia, the ulnar malunion is evident on radiograph; treatment requires corrective osteotomy of the ulna (not simple reduction).
• Miller's Review of Orthopaedics: "Proximal-third ulnar fracture accompanied by radial head subluxation/dislocation... anatomic reduction of the proximal ulna usually reduces the radial head."

5. Post-traumatic/Degenerative Elbow Arthritis with Valgus Deformity

• Severe post-traumatic arthritis (e.g., after old supracondylar fracture with malunion) can produce a cubitus valgus deformity with apparent lateral displacement of the radial head and loss of ROM, mimicking a dislocation on clinical examination.
• Unlike a true dislocation, radiographs show preserved (though distorted) joint articulation, osteophytes, joint-space narrowing, and periarticular heterotopic ossification.
• Heterotopic ossification (HO) around the elbow — a common complication after elbow trauma — can massively restrict motion and give the appearance of a fixed deformity even without a true dislocation.
• Sabiston Textbook of Surgery: Late complications after elbow dislocation include "chronic instability, stiffness, post-traumatic arthritis, and heterotopic ossification."

6. Pulled Elbow (Nursemaid's Elbow) — In Children

• In children, an annular ligament injury with radial head subluxation can become recurrent and mimic a partial posterolateral dislocation.
• Key distinction: typically <5 years age, mechanism of axial traction on the forearm, immediate refusal to use the arm, no bony deformity on X-ray.
• This does not persist as a true chronic dislocation in most cases.

7. Varus Posteromedial Rotatory Instability (VPMRI)

• VPMRI is caused by anteromedial coronoid fracture under a varus + posteromedial rotatory force — producing instability that partially overlaps with PLRI in clinical presentation (instability, limited ROM).
• However, the instability vector is posteromedial (not posterolateral), and the pivot-shift test is negative; the key finding is an anteromedial coronoid fracture on CT.
• Miller's Review of Orthopaedics: "Increasingly recognized but poorly understood entity; etiology: fracture of anteromedial coronoid process."

Summary Table

Investigations to Distinguish

1. Plain X-rays (AP + lateral) — First line; assess radial head morphology, presence of ulnar fracture, joint-space narrowing, heterotopic ossification.
2. CT scan — Essential to characterize fracture fragments (coronoid, radial head), rule out Monteggia malunion.
3. MRI — Best for ligament assessment (LUCL, MCL integrity); distinguishes reducible instability from fixed dislocation.
4. Examination under anesthesia (EUA) — The lateral pivot-shift test often requires EUA to reproduce PLRI without patient guarding.

• Rockwood & Green's Fractures in Adults, 10th ed., 2025
• Miller's Review of Orthopaedics, 9th ed.
• Bailey & Love's Short Practice of Surgery, 28th ed.